1. Field of Invention
This invention relates to an electrode structure for the production of aluminum by electrolysis of alumina dissolved in a molten cryolitic bath, and, more particularly, to a cathode of titanium diboride (TiB.sub.2) and other refractory hard metal materials, or mixtures of these materials, such as the refractory carbides and borides of the transition elements, titanium and zirconium, (hereinafter collectively referred to as RHM) in a novel and improved arrangement in electrical systems for electrolytic cells for producing aluminum. Further, the invention relates to a replaceable RHM electrode structure which is easily handled during preheating, installation in the cell and changing of the electrode during operation of the cell.
2. Description of the Prior Art
In the early 1950's, RHM materials were first utilized for cathode constructions in aluminum reduction cells. Titanium and zirconium borides and carbide boride mixtures were found suitable for these constructions, and various cathode constructions are shown in British Pat. Nos. 784,695; 784,696; 802,471 and 802,905 and in U.S. Pat. No. 3,028,324. This RHM cathode development is chronicled in a published paper identified as follows: C. E. Ransley, "The Application of the Refractory Carbides and Borides to Aluminum Reduction Cells," Extractive Metallurgy of Aluminum, Volume 2, Interscience Publishers, New York, (1963) p. 487. RHM materials in pure form are very resistant to the molten aluminum and cryolite found in an aluminum reduction cell and moreover generally have higher electrical conductivities than the conventional carbon products used in a reduction cell. In addition, RHM and in particular TiB.sub.2 are readily wet by molten aluminum, whereas the carbon products normally used are not.
Although the early use of RHM in aluminum reduction cells was conceptually a significant improvement, such use was fraught with practical problems and as a result the development of RHM cathodes has not met with any significant commercial success.
One major problem faced by the workers in this area was the deleterious effects of oxide in the RHM shapes used in the reduction cell. Normally, the RHM shapes were formed from RHM powder by either hot pressing or cold pressing and sintering. However, the surfaces of the RHM particles were oxidized to a certain extent so that when the powder was pressed into various shapes, a high concentration of oxide resulted at the interparticle or grain boundaries. The integranular oxide could be readily attacked by molten aluminum so that the RHM particles or grains could be easily dislodged after molten aluminum attack at the grain boundaries, resulting in the rapid deterioration of the protective RHM cathode surface. During the early development work on RHM cathode materials, it was well known that the oxide content of RHM shapes must be kept as low as possible to avoid intergranular attack by molten aluminum. However, the art of RHM manufacture was not sufficiently advanced at that time to produce high purity RHM products which could withstand attack by molten aluminum for any significant period. Theoretically, RHM with no oxide content would be best, but such material is impossible to obtain in a commercial process. Lately, several manufacturers have been able to produce TiB.sub.2 shapes of a reasonable size with oxide contents less than 0.05% by weight, which makes the TiB.sub.2 shapes very resistant to molten aluminum attack even at the grain boundaries where the oxide tends to be concentrated.
Because the RHM materials have a high elastic modulus and low Poisson's ratio, they are quite brittle and subject to thermal shock. As a general rule, RHM shapes should not be subjected to a temperature differential greater than 200.degree. C. to avoid thermal cracking. They are more tolerant to heating up than cooling down conditions.
A particularly attractive aluminum reduction cell design utilizing RHM cathodic surfaces is shown in U.S. Pat. No. 3,400,061, wherein the RHM cathode surfaces are sloped so that only a thin layer of molten aluminum which wets the RHM surface remains. The molten aluminum electrolytically formed during the operation of the cell drains from the sloped surface into the trough or trench located at the middle of the cell. The molten aluminum in the trough is not a part of the electrolytic circuit and can be removed as desired. Only the thin layer of molten aluminum which wets the RHM cathodic surface is involved in current transfer and permits electrolysis operation at a low interpolar or anode cathode distance (ACD) which reduces the energy loss due to the resistance drop in the electrolyte. A significant savings in energy (up to about 25%) would be realized by a low ACD, e.g., one-half inch, over the conventional reduction cells. However, in RHM cathode constructions wherein the RHM material is supported on a carbonaceous substrate, there is a significant problem in that there is an extremely large difference in thermal expansion between RHM shapes and the supporting conductive carbonaceous substrate. The large difference in thermal expansion coefficients (e.g., about 2.times.10.sup.-6 v. 8.times.10.sup.-6 in/in.degree. C.) precluded forming a bond which would be effective both during installation of the RHM shapes at room temperature and the operating temperature of the aluminum reduction cell (e.g., about 975.degree. C.). Any bond formed at room temperature when the plate or tiles of RHM were installed would be essentially destroyed by the thermal expansion during heatup to operating temperature.
The patents and technical literature are replete with references which describe attempts to solve the various problems in the use of TiB.sub.2 and other RHM in the harsh environments of an aluminum reduction cell. Lewis et al in U.S. Pat. No. 3,400,061 and others utilized a mixture of TiB.sub.2 and other refractory hard metals with small amounts of carbon to reduce the relatively large thermal expansion of the RHM materials. However, such composites did not have the service life necessary for commercial usage due to their susceptibility of attack by the electrolytic bath. References such as U.S. Pat. Nos. 2,915,442; 3,081,254; 3,151,053; 3,161,579; and 3,257,307 describe RHM cathode bars in various positions. However, the RHM cathode bars usually could not withstand the thermal distortion attendant with such designs and they inevitably fractured due to the brittleness of the RHM.
A recent development of RHM cathode design is that disclosed in U.S. Pat. No. 4,071,420, wherein an array of RHM parts or shapes, such as plates, bars, hollow cylinders and the like, are fastened or embedded at one extremity in the carbonaceous bottom of the cell, while the other extremity protrudes into the cryslitic bath and the parts are arranged preferably in a pattern of regularity beneath the anodic surface area of the carbon anode. However, these arrangements also have difficulties because of the brittleness of the RHM materials leading to a short life of the cathode members of the cell, necessitating premature shutdown of the cell for repairs. This causes a serious interruption of the productivity of the cell.
U.K. Published patent application No. 2,024,864 Jan. 16, 1980) discloses a wettable cathode element which is exchangeable and which is made of titanium carbide, titanium diboride or pyrolitic graphite. Although this cathode element can be replaced during operation of the cell, the subelements of the RHM material are of complex shapes having sharp angles and corners and require the joining by screws and the like. A structure such as proposed would be subject to cracking under the rigors of an electrolytic cell environment.
It has long been recognized that the principal energy loss of the Hall-Heroult cell is due to the resistive loss of the electrolyte in the interpolar gap or anode-cathode distance (ACD). At typical current densities, this drop is about one volt per inch which is 20 to 25 percent of the total cell voltage. Much effort has been expended to minimize the ACD, but commercial conventional cells must operate in excess of one and one-half inches. This requirement is due to the very strong inverse relationship between current efficiencies and ACD. Also, as the ACD is reduced toward one inch, the voltage becomes unstable. These effects are directly or indirectly the result of surface fluctuations of the molten aluminum pad which, in the conventional cell, is the cell cathode. The metal motion is attributable to electromagnetic forces and hydrodynamic forces. The latter are created by the anode gases emerging from the interpolar gap.
The prior art attempts to use RHM materials as cathode material for aluminum reduction cells have all suffered from practical deficiencies that prevent commercial use in Hall-Heroult cells, for example, the lack of achieving a long economic life, the catastrophic failure of the substrate when a localized RHM failure occurred, or the RHM cathode structure lacked dimensional stability thereby the spatial relationship of ACD could not be preserved. The use of RHM, e.g., titanium diboride, cathodes is governed by the economic balance between the cost savings realized from reduced power consumption and the high material cost, coupled with the associated capital investment. The already large capital investment in aluminum reduction smelters favors the retrofitting of cells with TiB.sub.2 cathodes rather than the replacement with a new cell design.